Despite considerable speculation the role of the eosinophil remains obscure. Observations of eosinophilia in parasitic infection and hypersensitivity reactions have contributed to the notion that these cells have an important role in the immune response. The objective of the proposed is to elucidate the biologic role of the eosinophil through study of the distinctive eosinophil granule. The focus will be the eosinophil peroxidase system, consisting of the unique granular enzyme eosinophil peroxidase, hydrogen peroxide generated by the cell, and an oxidizable co-factor, such as iodide. The function of eosinophil peroxidase will be investigated by measurement of the incorporation of I125 into protein (iodination) by intact cells and by purified peroxidase as well as localization of the site of iodination in intact cells. In order to assess the role of peroxidase in the microbicidal activity, the ability of eosinophils and purified peroxidase to iodinate the parasite Trichinella spiralis will be compared with microbicidal activity employing standard methods for determining larval and worm viability. The production of hydrogen peroxide will be studied under resting and stimulated conditions by fluorescence spectroscopy. The information derived should provide a basis for further understanding of the effector role of the eosinophil in tissue reactions and parasite killing.